In recent years, lithium-ion secondary batteries have been developed. Because of their high thermal stability, lithium-containing composite oxides having olivine structures, such as LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4, have been expected as positive electrode active materials of lithium-ion secondary batteries.
In order to increase the discharge capacity and the energy density of lithium-ion secondary batteries, attempts have been made to reduce the particle diameters and variation in particle size of active materials included in an active material layer that relates to intercalation and deintercalation of ions functioning carriers (see Patent Document 1).